Urban Green Roofs Research Articles

BP neural network multi-module green roof thermal performance prediction model optimized based on sparrow search algorithm

Green roofs, as a passive building energy-saving technology, offer a range of benefits including reduced energy consumption and mitigation of the urban heat island effect. To analyze the thermal performance of green roofs more accurately and efficiently, this study introduces a novel predictive model that utilizes a Sparrow Search Algorithm (SSA) optimized Backpropagation Neural Network (BPNN) for forecasting the thermal performance of green roofs in subtropical regions. Compared to traditional BPNN, the SSA-BPNN model enhances the network's convergence and generalization capabilities by optimizing initial weights and thresholds. The model, established using meteorological data from Guangzhou, China, and simulated green roof temperature data, underwent rigorous training and validation. The results indicate that predictive accuracy has significantly improved across all seasons, with substantial reductions in all 12 evaluation metrics, and the model GR3 (SSA-BP) emerging as the optimal model. Particularly in summer, the SSA-BPNN model exhibits the highest accuracy, underscoring its effectiveness for subtropical climates. The findings suggest that the SSA-BPNN model is a powerful tool for sustainable urban planning and green roof design in subtropical regions, marking a significant advancement in the integration of artificial intelligence in environmental engineering.

Towards promoting sustainability in the learning environment – a focus on green walls and green roofs

Environmental concerns have gained international attention as a result of rising urbanisation and population expansion. One of the main sectors responsible for the devastation of the environment is the construction industry. It is responsible for approximately 40% of global energy consumption and nearly 30% of greenhouse gasses. Aside from the energy crisis, climate change, global warming, declining biodiversity, and health issues are some of the challenges facing current society. Those have led to the amalgamation of natural vegetation into the built environment. Amongst several forms of natural vegetation such as rain gardens, street trees, urban drainage systems, green roofs, and green walls, etc., Due to the limited availability of land and the multiple advantages that it offers in terms of three sustainability pillars, green walls and roofs represent the most technologically sound solutions.

Green roof plant physiological water demand for transpiration under extreme heat

Green roofs are a sustainable strategy for improving the eco-environment in urban areas. However, plants on green roofs are increasingly threatened by extreme heat and drought due to climate change. There is a lack of comprehensive understanding of the physiological water demand of green roof vegetation under extreme heat. Therefore, this exploratory study investigated two commonly used Sedum species for urban green roofs (Sedum spectabile Boreau and Sedum alfredii Hance) under extreme heat, considering two treatments: irrigation and non-irrigation. The results indicated that under extreme heat, if without irrigation, the two Sedum species’ mean daily water consumption by transpiration was approximately 4.68 g per Sedum plant. However, under irrigation, the diurnal variation of stomatal conductance of the two Sedum species allowed the stomata to remain open throughout the day, resulting in an over-twentyfold increase in mean daily water consumption through transpiration (120.34 g per Sedum plant) and twofold increase in leaf area index. A random forest model showed that multi-environmental factors explained 67.53 % of the variability in stomatal behavior. Photosynthetically active radiation and soil moisture are the primary environmental factors that directly affect stomatal conductance. The irrigation induces stomatal opening throughout the diurnal cycle, ensuring the continuation of vegetation growth and the maintenance of physiological functions under extreme heat, which then contributes to keeping its eco-environmental functions and providing sustainable services, such as cooling and carbon sequestration. The findings of this study can guide planning and managing green roofs in urban areas that face extreme heat events.

Exploring perceptions, cognitive factors, and motivations: A study on green structures on residential rooftops

Urban greenhouse and green roof cultivation propose a nature-based solution to current socio-ecological challenges in urban ecosystems, as it offers several socio-ecological solutions and benefits. The interdisciplinary techniques for scientific explorations of cognition and perception towards urban green structures could not get the attention of the researchers across varying scientific disciplines, especially in the developing world. Addressing the personal factors involved in human cognition and its subsequent impact on perceptions will lay the foundation for the studies of the field. The study included questions about socio-demographic characteristics and socio-environmental motivations, as well as a set of images depicting potential future of green structures. The study was accomplished in Tabriz city of Iran with 375 participants. The findings showed that individuals prioritize certain types of rooftop green structures based on their personal characteristics and motivations. Additionally, cognitive differences were observed among individuals based on age, gender, and marital status, leading to perceptual differences towards green structures. Socio-environmental motivations activate perceptual responses within individuals, influencing their preferences for specific types of rooftop greenhouses. Understanding the response of individuals towards green structure types is crucial for effective planning and design. This knowledge can enhance the overall success and satisfaction of rooftop green structures projects, leading to greater wellness and a positive impact on the environment.

Unlock city-scale energy saving and peak load shaving potential of green roofs by GIS-informed urban building energy modelling

Urban green roofs have emerged as a significant trend in urban architecture worldwide, offering numerous benefits, including enhanced energy performance, improved urban microclimate, and public health. This study proposes a holistic framework for assessing the green roofs' energy-saving potential at the city scale, achieved by scaling up building-scale energy simulations to city-scale energy demands. Firstly, massive building information is collected using Geometric Information System (GIS) technologies. Subsequently, prototype buildings are generated to accurately represent the geometric characteristics of buildings at the city scale. Building performance simulation is further conducted considering three types of plants on roofs. A case study in Xiamen, China, demonstrates the effectiveness of the proposed framework to efficiently quantify the city-scale energy-saving potential of green roofs. By implementing green roofs in Xiamen, energy savings of 1.62–1.83% and peak load shaving of 1.10–1.63% can be achieved for the whole city. Overall, the proposed framework has the potential for widespread application in other cities with minor adjustments to accommodate variations in climate and building parameters.

Investigating crop performance on urban green roofs using hyperspectral data

This study explores the dynamic relationship between crop plant performance and variety of treatment effects (water levels, diversity, and Sedum) on extensive urban green roofs. Green roofs have gained recognition as a sustainable urban design strategy, offering benefits such as stormwater management, energy efficient, and improved aesthetic enhancements. However, environmental stressors can challenge plant growth and performance on these emerging extensive green roofs. To help cope with these stressors, certain planting techniques have been implemented on green roofs such as plants grown with Sedum and increasing diversity types. However, monitoring the effects of these treatments in a rapid, timely manner are still in its infancy. To monitor plant health (specifically bush beans (Phaseolus vulgaris)) on green roofs, this study utilizes hyperspectral remote sensing technology using two crucial optical parameters: red edge point (REP) and red well point (RWP) to offer new valuable insights into monitoring plant health on green roofs. The results found that both REP and RWP information can detect significant differences between the treatment effects of water treatment (high and low) (p = 0.006, p = 0.001) and plants with or without existence of Sedum (Crassulaceae) (p = 0.01, p = 0.07) but could not find significant difference in diversity effects (single crop or multiple crops) (p = 0.20, p = 0.45). The results of REP and RWP was also able to find that plants with Sedum under low watering treatments showed worse plant performance than plants without Sedum under low watering treatments (p = 0.006, p = 0.006). This provides insights that remote sensing technology can detect that there may have been a competition of resources under low watering treatments and that that REP and RWP has the potential for a future sophisticated green roof monitoring platform.

Earlier Flowering Phenology and Pollinator Visitation on Urban Green Roofs Compared to Ground-Level Gardens

Urban green space, green infrastructure, and horticultural installations are gaining recognition for their potential to foster biodiversity. Green roofs are challenging growing environments for plants, characterized by extreme substrate temperatures, high light intensity, limited moisture availability, and limited substrate depth. Plants have a variety of physiological responses to these unique conditions, but little is known about how green roof growing conditions affect ecological characteristics like plant flowering phenology. Meanwhile, studies are only just uncovering the degree to which green roofs can provide habitat and support urban pollinator biodiversity. We evaluated the flowering phenology and made in situ pollinator observations of 15 plant taxa growing both on green roof systems and at ground level in Denver, Colorado, over two growing seasons. We found that flowering phenology occurs substantially earlier on green roofs compared to ground level among the observed plant taxa and observed a greater number of pollinators on green roofs early in the season, compared to ground level, presumably due to the availability of floral resources among the observed plant taxa. We observed significantly higher substrate temperatures along with wider diurnal temperature amplitude during the growing season that may contribute to the observed phenological patterns. Divergence in flowering phenology between individual plants of the same species on green roofs and plants at ground-level may have implications for organisms that rely on floral resources in urban environments. Earlier flower initiation on green roofs may provide pollinators with unique foraging opportunities and aid targeted conservation where early-season floral resources are limited.

Microscale Investigation of Urban Heat Island (UHI) in Annaba City: Unveiling Factors and Mitigation Strategies

Cities are facing significant challenges related to climate change, particularly due to the increasing impact of the Urban Heat Island (UHI) phenomenon. The present study investigated the UHI phenomenon at the microscale in Annaba, Algeria. The research involved a multi-step approach, starting with on-site measurements of urban microclimate parameters, performed in downtown Annaba on 6 July 2023. The UHI intensity was quantified by comparing city-measured temperatures with rural surroundings. Thermal imaging is then used to empirically identify the contributing factors to UHI initiation at the microscale. The study employed the ENVI-met model to analyse mitigation strategies, manipulating parameters for six scenarios including the current design of the study area. Outputs were used to assess the impact of these strategies on air temperature, mean radiant temperature, relative humidity, and wind speed. The findings revealed an intense UHI effect in Annaba city with a peak difference of 6.9 °C, with practical implications for buildings, ground and roads, vehicles, air conditioners, and specific facade materials. Introducing urban vegetation, particularly urban trees and green roofs, proved highly effectiveness in mitigating the UHI in downtown Annaba. Urban trees demonstrated the most substantial impact, reducing temperatures by 1.9 °C at 1 p.m., while green roof temperature reductions ranged from 0.1 °C to 2 °C.

Vicissitudes and prospects of green roof research: a two-decade systematic bibliometric review

Urban green roofs have emerged as an innovative nature-based solution for enhancing urban sustainability and resilience. Based on the Web of Science Core Collection database, the scholarly papers on green roofs published in the past two decades were analyzed quantitatively and visualized using bibliometric techniques. The study scope covered research countries, institutions, main journals, crucial authors, highly cited documents, hot topics, and research frontiers. The 3210 publications mined from the bibliographic database on green roofs have increased progressively in the study period, with a significantly faster pace in recent years. The research subjects changed gradually over time, extending into more disciplines and becoming decidedly multidisciplinary. The notable bibliometric features registered a pronounced spatial concentration of research outputs in a small number of countries, regions, research institutions, journals, and researchers, mainly associated with developed metropolises. The number of publications and citation impacts identified the world’s top ten researchers and journals. Well-defined clusters depicted by knowledge graphs indicated a diverse range in the number and strength of inter-node connections for countries, institutions, researchers, and research topics defined by keywords. The research scope focuses on quantifying benefits and costs and optimizing designs to maximize biodiversity, hydrologic, thermal, and energy benefits as urban sustainability and climate-adaptation solutions. Emerging research should enhance biodiversity through optimal plant selection, integrate green roofs with renewable energy systems for sustainability, apply computational tools to enhance designs, evaluate low-cost and locally relevant design options for developing cities, and quantify co-benefits like stormwater reduction and energy savings. Multidisciplinary efforts leveraging simulations, ecosystem services valuation, and participatory community partnerships can advance context-appropriate green roof innovation and evidence-based policymaking to expand adoption in cities worldwide. Quantifying and communicating co-benefits are critical measures to drive wider implementation.

Land Cover Classification and Change in Porto, Portugal: Spatial Analysis for Urban Vegetation and Green Roof Adoption

There are increasing pressures to create multifunctional urban areas, which provide space for people as well as ecosystem services. We analyzed landcover classification and change for Porto, Portugal, using geographic information systems and remote sensing data. Landcover was classified into seven categories for five counties of Porto District Portugal using PlanetLabs PlanetScope satellite imagery. Images were selected for low cloud cover and similar time of year and obtained for 4 July 2017 and 1 July 2022. Political boundaries were obtained from the National Institute of Statistics. Of the four classification methods tested, K-Nearest Neighbor was the most accurate supervised classification method with 2017 Kappa = 0.87 and 2022 Kappa = 0.98. Classified landcovers show Shadow, Other Roofing Material, and Bare Ground increased, and Water, Tiled Roof, Pavement, and Vegetation decreased from 2017 to 2022. NDVI showed a decrease in vegetation overall, mean difference -0.043. Classification showed 142.5 million square meters classified as impervious surface in 2022, of which 32.68 million square meters were “other roofs”, commercial or industrial white surfaced roofs, which may be the most likely targets for vegetative interventions. Zonal statistics show neighborhoods that gained or lost vegetation over the study period, and these neighborhoods are not randomly distributed, Moran’s I = 0.0249 or 0.402. Additional socioeconomic data could help understand this distribution and make decisions addressing climate justice. LiDAR data would increase the accuracy further and allow for clearer selection of roofs and quantification of green roof benefits.

CO2 removal characteristics of a novel type of moss and its potential for urban green roof applications

The feasibility of a novel type of moss (Parkortanso No. 1 synthesized from Racomitrium japonicum, Dozy and Molk) to capture CO2 in urban areas was demonstrated. The effects of light intensity (500, 1000, and 1500 µmol/m2.s), ambient temperature (10 °C, 25 °C, and 35 °C), age (1-year-old and 3 years old), and leaf color (bright and dark green) on the CO2 removal caused by the moss concerned were investigated. It was determined that stronger light intensity resulted in higher CO2 removal by the target moss. The moss showed the best CO2 capture at 25 °C, while the CO2-capturing capacities declined when the ambient temperatures were 10 °C and 35 °C. Three years old bright green moss was found to have higher CO2-capturing capacity than 1 year old. Similarly, bright green moss exhibited the best CO2 uptake out of the mosses concerned. The highest net CO2 emission of the moss was − 1.94 ± 0.72 kgCO2/m2.year, which was comparable to other moss and plant species. Consequently, the bright green and old Parkortanso No. 1 moss are recommended for a green roof application in terms of CO2 capture.Graphical

Urban ecosystem services and climate change: a dynamic interplay

Urban ecosystems play a crucial role in providing a wide range of services to their inhabitants, and their functioning is deeply intertwined with the effects of climate change. The present review explores the dynamic interplay between urban ecosystem services and climate change, highlighting the reciprocal relationships, impacts, and adaptation strategies associated with these phenomena. The urban environment, with its built infrastructure, green spaces, and diverse human activities, offers various ecosystem services that enhance the wellbeing and resilience of urban dwellers. Urban ecosystems offer regulatory services like temperature control, air quality upkeep, and stormwater management, plus provisioning like food and water. They also provide cultural benefits, promoting recreation and community unity. However, climate change poses significant challenges to urban ecosystem services. Rising temperatures, altered precipitation patterns, and increased frequency of extreme weather events can disrupt the functioning of urban ecosystems, impacting the provision of services. Heatwaves and urban heat island effects can compromise human health and energy demands, while changes in rainfall patterns can strain stormwater management systems and lead to flooding. Moreover, climate change can disrupt biodiversity and ecological processes, affecting the overall resilience and sustainability of urban ecosystems. To address these challenges, cities are adopting various adaptation strategies that recognize the interdependence between urban ecosystems and climate change. Green infrastructure interventions, such as the creation of urban parks, green roofs, and community gardens, aim to mitigate the impacts of climate change by enhancing the regulation of temperature, improving air quality, and reducing stormwater runoff. Additionally, urban planning and design approaches prioritize compact and walkable neighborhoods, promoting public transportation and reducing reliance on fossil fuels. Furthermore, engaging communities in the management of urban ecosystems and climate change adaptation measures is crucial for ensuring equitable distribution of ecosystem services and building social resilience. Therefore, the review article highlights a comprehensive understanding of the dynamic interrelationship between urban ecosystem services and climate change and their implications. By recognizing and integrating the contributions of urban ecosystems, cities can develop sustainable and resilient strategies to mitigate and adapt to climate change, ensuring the wellbeing and habitability of urban environments for present and future generations.

Thermal properties of green, white and other building roof materials and solar insolation: A case study in New York City

This report presents results from a case study of surface temperatures of an urban green roof, a white roof, and other roof surfaces, at Hunter College, of the City University of New York. The analysis includes manual as well as automated observations between 2012 and 2016. The cooling effect, defined as the surface temperature differences between the green and white surfaces compared to a black surface, are often greater than 20 °C, and sometimes greater than 30 °C for both green and white roofs. Solar insolation is found to be the best predictor of the cooling effect, explaining over 72% and 77% of the variance for green and white roofs (respectively), with a 2-3 °C enhanced cooling effect for every 100 W/m2 increase in solar insolation. The response times of both green and white roofs to solar insolation is estimated to be 20 min (±10 min). Green roofs are found to have a slight cooling advantage (∼1.7 °C) over white roofs at higher solar insolation (∼1000 W/m2) and higher surface temperatures (∼45 °C). Key components of the surface energy balance are considered to help explain the observed thermal behavior of the green and white roofs. Manual observations, instrument maintenance, as well as data transcription and quality control, were performed largely by undergraduate students, demonstrating that green infrastructure can provide valuable learning experiences that may be difficult to otherwise provide in urban environments.

Native plants on experimental urban green roofs support higher community-level insect abundance than exotics

Urbanization is one of the multiple factors leading to global insect declines. As urbanization grows, green roofs represent a nature-based solution that could provide habitat for plants and animals within cities. Several characteristics of green roofs could enhance insect biodiversity. Nevertheless, little is known about the effects of plant origin on insect abundance from a multi-taxa perspective. Thus, our main goal was to evaluate the effects of plant origin, local resources, and urbanization level on green roofs’ overall insect abundance, the abundance of different taxonomic orders, and community composition. Using experimental green roofs, we compared insect communities between native and exotic plants across an urbanization gradient, in Córdoba city, central Argentina. On the roofs of 30 houses, we installed two blocks of a modular extensive green roof system, with either native or exotic plant species (6 species each). In March 2019, we used yellow pan traps and pitfall traps (N = 360) to sample insects and measured flower abundance and plant cover at each plant treatment. The urbanization level of each house was estimated by the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Built-up Index (NDBI), and the Land Surface Temperature (LST). A total of 35,257 insects from 12 orders were registered, with Diptera, Hymenoptera, and Hemiptera as the dominant groups. Native plants supported significantly higher total insect abundance on both types of traps, independently from the urbanization level, flower abundance, or plant cover of each roof. The abundance of most of the taxonomic orders was higher in natives as well, but particular effects of the covariables were detected for certain groups. In addition, plant origin was an important factor for insect composition. Our results highlight that green roof design should prioritize native plants to enhance insect conservation while achieving more sustainable cities.

Water consumption of three ornamental species with the suction irrigation system

Objective: To evaluate water consumption in ornamental plants (geranium, gazania and petunia) in two substrates with different particle sizes (fine and coarse) of tezontle and peat moss, through a suction irrigation system, which uses porous capsules as irrigation emitters. Design/methodology/approach: The experimental design was in complete randomized blocks, with six treatments (three ornamental species and two substrates) with four repetitions (24 experimental units). The first mixture contains fine substrate (composed of tezontle with particle size less than 0.4 mm) and peat moss, in a 1:2 v/v ratio; the second mixture presents coarse substrate (composed of tezontle with particle size between 0.4-0.6 mm) and peat moss, 1:1 in v/v. Results: In water consumption, there are significant differences by ornamental species and type of substrate, where the irrigation system has the ability to self-regulate. The highest water consumption was in petunia in the coarse substrate (which has a higher proportion of peat moss in its composition). Limitations on study/implications: With use in protected agriculture, rural and urban orchards, gardens, walls and green roofs and research purposes. Findings/Conclusions: The suction irrigation system through porous capsules has the capacity to continuously supply the water demanded by the plant-substrate-atmosphere system.

Nature-Based Solutions for Carbon Neutral Climate Resilient Buildings and Communities: A Review of Technical Evidence, Design Guidelines, and Policies

The building sector is responsible for nearly 40% of the total global direct and indirect CO2 emissions. Urban green infrastructure, which includes features such as urban trees, vegetation, green roofs, and green facades, are examples of nature-based solutions often employed as municipal climate mitigation and adaptation strategies. This approach offers a range of cost-effective strategies for reducing municipal CO2 emissions and presents compelling public policy co-benefits such as improved urban livability and enhanced environmental conditions. For municipalities to confidently deploy these solutions at a scale necessary to achieve climate benefits, acquiring knowledge of quantifiable and demonstrated outcomes is an essential requirement. The objectives of this paper are to (1) provide a comprehensive analysis of the advantages and limitations of nature-based solutions (NBS) to address the challenge of reducing CO2 emissions; (2) evaluate existing design guidelines and policies as may be available across Canada, and that that support the of implementation of NBS in urban agglomerations; (3) identify knowledge gaps and research needs to address challenges to the implementation of NBS. In this review, suggestions and requirements as presented in these documents are examined while giving due consideration to the scientific evidence available in research papers. It was found that the adoption of NBS can contribute to carbon neutral communities through reduced building energy consumption and carbon sequestration. Supportive guidelines and policies have been developed, or are in development, to promote the implementation of NBS at the city scale, despite challenges in assessing, quantitatively, their impact due to uncertainties in data, methods, and scale. Nonetheless, existing research provides sufficient evidence to qualify the measures and suggestions outlined in the guidelines and policies described in this paper.

Sustainable Growth of Medicinal and Aromatic Mediterranean Plants Growing as Communities in Shallow Substrate Urban Green Roof Systems

To date, the evaluation of the performance of Mediterranean native plants under urban green roof conditions has been limited to monoculture studies. However, plants grow naturally in plant communities and therefore it is of interest to evaluate their growth under realistic conditions, including interactions among plant species. The present study aims to evaluate the growth, flowering, and self-reproduction capacity of three artificially created plant communities consisting of native Mediterranean plants, as exemplified in Greece under shallow green roof substrate depths (8 and 15 cm) and two irrigation regimes (high, 20% ETo and low, 10% ETo). The plant communities (PC) were designed to resemble xerophytic vegetation found either in Chania, Crete (PC-1), as a combined pattern with plants from Attica, Crete, and the Cyclades (Kythnos) (PC-2), or to resemble the coastal vegetation of Attica and Cyclades (Kythnos) (PC-3). Each of the three artificial plant communities (PC-1, PC-2 and PC-3) consisted of nine species and subspecies. The deeper substrate significantly improved the growth, flowering and survival of most plant taxa. The irrigation regime was not significant for all species except one, indicating that minimal amounts of irrigation are required. Four species did not manage to bloom while 15 species were able to self-reproduce.

Rooftops for Whom? Some Environmental Justice Issues in Urban Green Roof Policies of Three North American Cities

Green roofs provide multiple benefits to households and urban neighbourhoods by lowering temperatures, reducing energy costs, and reducing stormwater runoff. However, green roofs are expensive and may not be available to all residents, which raises questions about who benefits from green roofs and urban policies that support this technology. Policies to support different social groups’ access to this green infrastructure is important for achieving equity. This study aims to identify and analyze to what extent existing different rooftop greening incentives and mandatory policies in three American cities, including New York, Chicago, and Philadelphia, address environmental justice. Nine policies were analyzed from three cities based on three dimensions of environmental justice. The results show that all policies have neglected a degree of distributional, recognitional, and procedural environmental justice in their policies. Although incentives try to advance distributional justice by implementing green roofs in different parts of the city, these policies do not pay enough attention to support themes such as zoning, size, rental status, land value, and affordability in all urban areas.

Mapping environmentally sustainable urban development within six US cities through object-based image change analysis of aerial orthoimagery

The rapid expansion of cities and continuous urban population growth underscores a need for sustainable urban development. Sustainable development is that which addresses human needs, contributes to well-being, is economically viable, and utilizes natural resources at a degree sustainable by the surrounding environmental systems. Urban green spaces, green roofs, and solar panels are examples of environmentally sustainable urban development (ESUD), or development that focuses on environmental impact, but also presents the potential to achieve social and economic sustainability. The aim of this study was to map and compare amounts of ESUD c. 2010 and c. 2019 through an object-based image analysis (OBIA) approach using National Agricultural Imagery Program (NAIP) aerial orthoimagery for six mid- to large-size cities in the USA.The results of this study indicate a hybrid OBIA and manual interpretation approach applied to NAIP orthoimagery may allow for reliable mapping and areal estimation of urban green space and green roof changes in urban areas. The reliability of OBIA-only mapping and estimation of areal extents of existing green roofs, and new and existing solar panels, is inconclusive due to low mapping accuracy and coarse spatial resolution of aerial orthoimagery relative to some ESUD features. The three urban study areas in humid continental climate zones (Dfa) were estimated to have greater areal extent of new and existing urban green space and existing green roofs, but less areal extent of new green roofs and existing solar panels compared to the three study areas in humid subtropical climate zones (Cfa).

Urban green roofs can support a diversity of parasitoid wasps

Green roofs are often installed atop buildings to provide ecological services such as mitigating storm water runoff and cooling air within urban heat islands. We found that green roofs in Portland, Oregon, also can support biodiversity, including a diverse assemblage of parasitoid wasps, with 20 morphospecies from 10 families present on the four roofs we surveyed. The roofs with greater plant diversity and structural complexity harbored comparatively more parasitoid morphospecies than the structurally simpler Sedum-dominated roofs. The oldest green roof supported much greater diversity than the younger roofs, including a comparably planted roof three times its size. Parasitoid wasps from the Hymenopteran families we found are high-trophic-level organisms known to feed on a variety of arthropods, including many insects commonly considered to be pests in urban areas. We suggest that green roofs have the potential to provide an often-overlooked ecological service by supporting parasitoid wasps that can act as natural biological control agents.